High-tensile steel which is more highly strong and highly ductile is demanded for the purpose of securing car crash safety and a weight reduction of an automobile both at a high level. As framework parts and components of a car body in particular become thinner, car crash safety based on an improved strength is increasingly important.
To be particularly noted these days is accelerated promotion of a weight reduction based on use of high-tensile steel in an attempt to meet the COP3 requirement (International Conference on the Prevention of Global Warming of 1997), emission control regulations (becoming effective in 2008 in Europe and in 2009 in Japan). Further, with tightening of regulations regarding a crash to the side sections of a car (including tightening of the US Safety Standard in 2005 for instance), the demand is rising for steel which is more highly tensile (e.g., super-high-tensile steel whose tensile strength TS is 780 MPa or greater). A high-strength steel sheet nevertheless must also be excellent in formability: for various applications, appropriate formability is required.
However, members, pillars and the like affecting car crash safety for instance among parts and components which are used in a car body have particularly complex shapes, which leaves a problem that it is not possible to ensure proper formability with the mechanical properties (such as TS×EL=14700 MPa·% where the tensile strength TS is 980 MPa and the elongation EL is 15%) of conventional DP steel (Dual-phase steel).
Meanwhile, TRIP (Transformation Induced Plasticity) steel sheets are gaining a renewed attention as high-strength steel sheets which are excellent in elongation. A TRIP steel sheet is a steel sheet in which an austenite structure remains present and which significantly elongates as the residual austenite (γR) is induced to transform into martensite due to stress when processed and deformed at a temperature equal to or higher than the martensitic transformation start temperature (Ms point). Known as such includes TRIP-type complex-structure steel (TPF steel) whose main phase is polygonal ferrite and which contains residual austenite, TRIP-type bainitic steel (TBF steel) whose mother phase is bainitic ferrite and which contains residual austenite, etc.
Of these, TBF steel has long been known (NISSHIN STEEL TECHNICAL REPORT, No. 43, December 1980, pp. 1-10) and makes it easy to attain a high strength because of its hard bainitic structure. It is characterized in exhibiting extremely favorable elongation (total elongation) since very fine residual austenite tends to be created at the boundary of lath bainitic ferrite in the bainitic structure. Another advantage of TBF steel is an advantage related to manufacturing that TBF steel is easily produced through one thermal processing (continuous annealing or plating).
However, although being excellent in total elongation (EL), conventional TBF steel is not yet satisfactory with respect to uniform elongation. Although the uniform elongation (u-EL), which is important to improve the punch stretch formability, needs be excellent particularly in the case of members, pillars and the like mentioned above which are components requiring stretch forming, TBF steel proposed so far does not have excellent uniform elongation, and therefore, there is a serious need for a further improvement of this characteristic.